json-0.5: Support for serialising Haskell to and from JSON

MaintainerSigbjorn Finne <sof@galois.com>
Safe HaskellSafe-Infered



JSON serializer and deserializer using Data.Generics. The functions here handle algebraic data types and primitive types. It uses the same representation as Text.JSON for Prelude types.



module Text.JSON

class Typeable a => Data a

The Data class comprehends a fundamental primitive gfoldl for folding over constructor applications, say terms. This primitive can be instantiated in several ways to map over the immediate subterms of a term; see the gmap combinators later in this class. Indeed, a generic programmer does not necessarily need to use the ingenious gfoldl primitive but rather the intuitive gmap combinators. The gfoldl primitive is completed by means to query top-level constructors, to turn constructor representations into proper terms, and to list all possible datatype constructors. This completion allows us to serve generic programming scenarios like read, show, equality, term generation.

The combinators gmapT, gmapQ, gmapM, etc are all provided with default definitions in terms of gfoldl, leaving open the opportunity to provide datatype-specific definitions. (The inclusion of the gmap combinators as members of class Data allows the programmer or the compiler to derive specialised, and maybe more efficient code per datatype. Note: gfoldl is more higher-order than the gmap combinators. This is subject to ongoing benchmarking experiments. It might turn out that the gmap combinators will be moved out of the class Data.)

Conceptually, the definition of the gmap combinators in terms of the primitive gfoldl requires the identification of the gfoldl function arguments. Technically, we also need to identify the type constructor c for the construction of the result type from the folded term type.

In the definition of gmapQx combinators, we use phantom type constructors for the c in the type of gfoldl because the result type of a query does not involve the (polymorphic) type of the term argument. In the definition of gmapQl we simply use the plain constant type constructor because gfoldl is left-associative anyway and so it is readily suited to fold a left-associative binary operation over the immediate subterms. In the definition of gmapQr, extra effort is needed. We use a higher-order accumulation trick to mediate between left-associative constructor application vs. right-associative binary operation (e.g., (:)). When the query is meant to compute a value of type r, then the result type withing generic folding is r -> r. So the result of folding is a function to which we finally pass the right unit.

With the -XDeriveDataTypeable option, GHC can generate instances of the Data class automatically. For example, given the declaration

 data T a b = C1 a b | C2 deriving (Typeable, Data)

GHC will generate an instance that is equivalent to

 instance (Data a, Data b) => Data (T a b) where
     gfoldl k z (C1 a b) = z C1 `k` a `k` b
     gfoldl k z C2       = z C2

     gunfold k z c = case constrIndex c of
                         1 -> k (k (z C1))
                         2 -> z C2

     toConstr (C1 _ _) = con_C1
     toConstr C2       = con_C2

     dataTypeOf _ = ty_T

 con_C1 = mkConstr ty_T "C1" [] Prefix
 con_C2 = mkConstr ty_T "C2" [] Prefix
 ty_T   = mkDataType "Module.T" [con_C1, con_C2]

This is suitable for datatypes that are exported transparently.


Data Bool 
Data Char 
Data Double 
Data Float 
Data Int 
Data Int8 
Data Int16 
Data Int32 
Data Int64 
Data Integer 
Data Ordering 
Data Word 
Data Word8 
Data Word16 
Data Word32 
Data Word64 
Data () 
Data Handle 
Data DataType 
Data ThreadId 
Data TypeRep 
Data TyCon 
Data ByteString 
Data ByteString 
Data IntSet 
Data a => Data [a] 
(Data a, Integral a) => Data (Ratio a) 
Typeable a => Data (StablePtr a) 
Typeable a => Data (IO a) 
Typeable a => Data (Ptr a) 
Typeable a => Data (ForeignPtr a) 
Typeable a => Data (STM a) 
Typeable a => Data (TVar a) 
Typeable a => Data (IORef a) 
Typeable a => Data (MVar a) 
Data a => Data (Maybe a) 
Data a => Data (IntMap a) 
(Data a, Ord a) => Data (Set a) 
(Data a, Data b) => Data (a -> b) 
(Data a, Data b) => Data (Either a b) 
(Data a, Data b) => Data (a, b) 
(Typeable s, Typeable a) => Data (ST s a) 
(Typeable a, Data b, Ix a) => Data (Array a b) 
(Data k, Data a, Ord k) => Data (Map k a) 
(Data a, Data b, Data c) => Data (a, b, c) 
(Data a, Data b, Data c, Data d) => Data (a, b, c, d) 
(Data a, Data b, Data c, Data d, Data e) => Data (a, b, c, d, e) 
(Data a, Data b, Data c, Data d, Data e, Data f) => Data (a, b, c, d, e, f) 
(Data a, Data b, Data c, Data d, Data e, Data f, Data g) => Data (a, b, c, d, e, f, g) 

toJSON :: Data a => a -> JSValueSource

Convert anything to a JSON value.

fromJSON :: Data a => JSValue -> Result aSource

Convert a JSON value to anything (fails if the types do not match).

encodeJSON :: Data a => a -> StringSource

Encode a value as a string.

decodeJSON :: Data a => String -> aSource

Decode a string as a value.